Remote communication utilizing wireless equipment typically relies on radio frequency (RF) technology, which is employed in many industries. One application of RF technology is in locating, identifying, and tracking objects, such as animals, inventory, and vehicles.
RF identification (RFID) tag systems have been developed to identify, monitor, or control remote objects.
An advantage of RFID systems is the non-contact, non-line-of-sight capability of the technology. Tags can be read through a variety of substances such as snow, fog, ice, paint, dirt, and other visually and environmentally challenging conditions where bar codes or other optically-read technologies would be useless. RF tags can also be read at remarkable speeds, in most cases responding in less than one hundred milliseconds.
There are three main categories of RFID tag systems. These are systems that employ beam-powered passive tags, battery-powered semi-passive tags, and active tags. Each operates in fundamentally different ways. The invention described below in the Detailed Description can be embodied in any of these types of systems.
The beam-powered RFID tag is often referred to as a passive device because it derives the energy needed for its operation from the radio frequency energy beamed at it. The tag rectifies the field and changes the reflective characteristics of the tag itself, creating a change in reflectivity (RF cross-section) that is seen at the interrogator. A battery-powered semi-passive RFID tag operates in a similar fashion, modulating its RF cross-section in order to change its reflectivity that is seen at the interrogator to develop a communication link. Here, the battery is the only source of the tag's operational power. Finally, in the active RFID tag, both the tag and reader have transceivers to communicate and are powered by respective batteries.
A typical RF tag system will contain at least one tag and one interrogator. The range of communication for such tags varies according to the transmission power of the interrogator, interrogator receiver sensitivity and selectivity, and backscatter characteristics of the tag. Battery-powered tags operating at 2,450 MHz have traditionally been limited to less than ten meters in range. However, devices with sufficient power can reach in excess of 100 meters in range, depending on the frequency and environmental characteristics.
Conventional continuous wave backscatter RF tag systems utilizing passive (no battery) RF tags require adequate power from a signal from the interrogator to power the internal circuitry in the tag used to modulate the signal back to the interrogator. While this is successful for tags that are located in close proximity to an interrogator, for example less than three meters, this may be insufficient range for some applications, for example greater than 100 meters.
A coplanar waveguide is a transmission line that shares some characteristics with microstrip lines. The characteristic impedance of a coplanar waveguide transmission line is determined by the distributed inductance and the distributed capacitance from the strip to the adjacent groundplane. In a grounded coplanar waveguide, some of the fields go through air, and (ideally) only a small fraction leak to a groundplane. Because some of the fields are in air, there is less loss. Tuning of the dielectric while the circuit is on is possible with no risk of shorts. Large metal top surfaces improve heat sinking, and because the waveguide is grounded, metal and screws can be added for even more heatsinking. The coplanar waveguide can be used to mount components in series and to shunt without need for drilling or use of plated through holes. This makes some circuits possible which would not be possible using plated through holes, if the inductance of plated through holes to the groundplane would be too high. Frequency multipliers are easily used with coplanar waveguides because there is a topside ground to mount diodes in shunt.
Readers or interrogators with good range have been developed by the assignee of the present invention using off-the-shelf packaged and connectorized components coupled together with coaxial interconnects. These readers have very long ranges, but are generally large, stationary, expensive units. A smaller, less expensive unit, with improved manufacturability, is desired.